Training on multi-slice v6e discovers MegaScale twice again

[tengyifei]

🐛 Bug

c.f. the tests in AI-Hypercomputer/torchprime#146, we can train on 2 slices with 2.6 stable, but any nightly version I tested all fails. The proximate cause is that tracing the training loop triggers MegaScale device discovery a second time (likely within flash attention). We've fixed this once in #8609, but it's failing again.

To Reproduce

Run checks on the PR AI-Hypercomputer/torchprime#146 with a nightly docker image of your choice.

Expected behavior

Training works on 2 slices.

Additional context

We fixes one source of double MegaScale discovery in #8609

[bhavya01]

This should be fixed by #8819 but testing is currently blocked due to lack of a healthy cluster.